Our goals are to understand the normal mechanisms by which the liver forms bile. The ultimate aim is to prevent liver injury from impairing this vital function which otherwise results in the clinical syndrome of cholestasis (clinical manifestations of bile secretory failure). This work focuses on two major areas: (1) The role of transport enzymes, and particularly sodium potassium ATPase, in the generation of bile secretion. (2) The role of the intercellular barrier (the junctional complex) which seals off the lumen of the bile canaliculus (the primary channel into which bile is secreted). Current studies use histochemical techniques, previously established, which localize the enzyme to the baso-lateral membrane of the hepatocytes. Drugs which both stimulate and inhibit sodium potassium ATPase in vivo and in vitro will be used to determine if there are alterations in the lobular distribution of the transport enzymes. Techniques will be developed for use in isolated plasma membrane fractions. The permeability characteristics of the junctional complex between hepatocytes will be studied in more detail. This intercellular barrier will be assessed by measuring the biliary clearance of radiolabeled insulin and sucrose, two large molecular weight solutes, will presumably enter bile through this pathway. The permeability of the barrier will be further assessed by electron microscopes using ionic lanthanum chloride, an electron-dense material which penetrates permeable junctions but does not enter tight junctions. Finally, freeze fracture replicas will be prepared and the junctional elements that make up the structural barrier will be assessed quantitatively. Studies will be performed in both portal and central zones of the lobule to determine if there are anatomical differences.